A dental handpiece for cutting a root canal(s) of a tooth (teeth) is constructed so as to rotate a cutting tool using a motor.
Here, the cutting tool for cutting a root canal of a tooth has a slender configuration. At the occasion of cutting the root canal of a tooth, if the cutting tool is jammed into the root canal of the tooth, the cutting tool undergoes a force in a twisting direction, which results in a problem that the cutting tool becomes apt to be broken.
Hence, there is proposed a method that prevents a cutting tool from being jammed into a tooth (teeth) and from being damaged by causing the cutting tool to work at a forward-reverse rotation mode in which it is rotated in one direction for a certain time and thereafter is rotated in opposite direction (hereinafter, occasionally simply called as “forward-reverse rotation”) (refer to, for example, Patent Document1).
In a conventional method of implementing forward-reverse rotation, as shown in FIG. 14, a voltage is applied to a motor (step S101), and the voltage is increased until the rotation speed of the motor reaches a set speed (steps S102, S103). After the rotation speed of the motor has reached the set speed, a relay is switched and the application direction of the voltage is switched at every lapse of a certain time (steps S104 to S107). The motor is thereby reversely rotated at every certain time, and the forward-reverse rotation of the cutting tool is implemented.